Known optical attenuation systems used, for example, to adjust light levels in optical communications systems, comprise absorption filters, metallic masks, integrating spheres, partially-reflective (or transmissive) windows or surfaces, diffraction elements or crossed polarizers. Unfortunately, such attenuation systems do not provide a stable output when used with light sources having high power levels.
Although they offer a high level of attenuation, mostly insensitive to input source beam shape, a disadvantage of the above-mentioned attenuation systems is that the degree of attenuation is not stable due to temperature effects resulting from heat generated in the body of the attenuation elements by the high energy light being received. For even higher energy levels, the heat may be sufficient to cause permanent damage to the attenuation elements.
To overcome, or at least mitigate, the heating effects, complex cooling mechanisms or systems may be devised at the expense of cost, size and reliability. For example, the problem of variations due to heating in attenuation systems which use adjustable diffraction elements, of the kind disclosed in U.S. Pat. No. 4,561,721 (Keilmann et al.), were addressed in U.S. Pat. No. 5,202,794 (Schnee et al.) by moving the diffraction element continuously so as to allow for cooling of a part that had just been irradiated. This arrangement is not entirely satisfactory, however, because the adjustment mechanism increases expense and reduces reliability.
Another approach that someone skilled in the art may readily use to obtain very high power detection is to limit the aperture size at the detector, for example by using an aperture in a screen in front of the detector, thus capturing only part of the light. Unfortunately, however, this technique is very sensitive to the input source numerical aperture and its position in relation to the aperture-detector combination.
An object of the present invention is to mitigate these disadvantages of known attenuation systems and to provide an optical attenuation system better suited to the detection of high energy light beams.